This invention relates generally to electrical grounding systems for aircraft, marine vessels, or ground vehicles to discharge static accumulation prior to fuel transfer, and particularly to an electronic circuit for detecting a particular grounding configuration and controlling fuel transfer in aircraft self-service fueling.
It is known that aircraft, marine vessels, and ground vehicles can accumulate significant electrostatic charges during transport. It is also known that petroleum distillates and other hydrocarbon fluids will themselves accumulate significant electrostatic charges when the fluids are transferred along the surface of a solid such as through insulated conduits or within storage tanks and processing equipment. Consequently, a significant hazard of explosion or fire is presented by the possibility of electrostatic discharge occurring proximate to a fuel reservoir, especially during fuel transfer or in preparation for fueling vehicles.
The accumulated electrostatic charge potential in aircraft, marine vessels, or ground vehicles depends on many variables, including their mass, the quantity and molecular properties of the contained fluid, the dielectric properties of that fluid and the reservoir, the intensity and duration of any movement between the fluid, container, vehicle, and environment, and the relative rates of accumulation and dissipation of that static charge In comparison, relatively accurate calculations regarding the accumulation and dissipation of electrostatic charges have been made for fuel storage tanks and fuel transfer conduits. Representative examples of systems for monitoring the accumulation or controlling the discharge of electrostatic charges in fuel storage tanks, fuel transfer conduits, and fuel metering systems include U.S. Pat. Nos. 3,141,113 and 3,160,785 to Munday, 3,453,493 to Godwin, 3,013,578 to Askevold, 2,315,805 to Mayo, 2,953,147 to Hornback, 3,821,603 to De La Cierva, 3,164,747 to Yahnke, and 3,784,876 to De Gaston. These systems include methods for the electrical grounding of the fluid and reservoir or conduit, controlling fluid flow rates as a function of accumulated electrostatic charge, neutralizing the charge using ionizing radiation or a counteracting electromagnetic field, and other means such as anti-static additives. In general, these systems are relatively complex and expensive, and are unsuitable for use in connection with a mobile reservoir such as an aircraft, marine vessel, or ground vehicle.
Various simple grounding systems have been devised to prevent the accidental or untimely discharge of an accumulated electrostatic charge in the proximity of a volatile inflammable fuel reservoir in ground vehicles such as tank trucks U.S. Pat. No. 1,600,549 to Jurs discloses the method of connecting a metal grounding chain to the reservoir and dragging the opposing end of that chain on the ground as the vehicle travels to dissipate the accumulated electrostatic charge U.S. Pat. No. 1,564,855 to Jurs discloses grounding both the fuel supply hose and the fuel tank in parallel through a common grounding wire, as well as grounding the vehicle using the chain disclosed in the Jurs '549 patent. The Jurs '855 patent provides a release mechanism which prevents opening the fill cap until the reservoir grounding wire has been connected U.S. Pat. No. 1,524,423 to Chapman discloses grounding the reservoir and the fluid within the reservoir in series through a common grounding wire. Various safety-related improvements have also been made in the grounding wires and mechanical couplings between those grounding wires and the fuel reservoirs, representative examples of these types of improvements being shown in U.S. Pat. Nos. 2,911,607 to Booth and 1,749,179 to Davis. It is also known to make the fuel supply hose electrically conductive, whereby accumulated electrostatic charges will be conducted to a grounded frame member, as shown in U.S. Pat. No. 1,230,356 to Wheaton.
While these systems have generally proven effective for their intended uses, they do present some drawbacks. It is incumbent upon the operator to ensure that a proper grounding connection is established prior to any fuel transfer, and the process of physically connecting the grounding wires in close proximity to the open fuel inlet port of the reservoir may produce a electrostatic discharge that will ignite fuel or vapors remaining within the reservoir.
Specialized requirements for aircraft fueling stations are mandated pursuant to the authority of the Federal Aviation Administration and airport regulations. Although current regulations do not encompass a mandatory grounding system or dictate equipment specifications for fuel dispensing systems, it is incumbent upon the operator of a fueling station to ground both the aircraft and the fuel supply hose prior to initiating fueling. The grounding configuration most frequently utilized comprises dual grounding cables for the aircraft and fuel supply hose. The aircraft grounding cable may be coupled to the frame or fuselage of the aircraft using a clamp, or a pin connector attached to the distal end of the grounding cable and a mating receptacle mounted on the aircraft frame or fuselage. One representative example of such a pin and receptacle assembly for use in aircraft grounding is shown in U.S. Pat. No. 4,541,684 to Holman. The fuel supply hose may be grounded using a separate grounding cable attached to the fuel hose or nozzle, or may include an insulated but internally grounded fuel hose which grounds either the fuel hose or the nozzle or both. If the nozzle or fuel hose are electrically conductive and contact the fuel inlet port on the aircraft (or a portion of the fuel reservoir within the aircraft), those elements may additionally be grounded by the fuel hose grounding cable. Representative examples of grounded fuel supply hoses are shown in U.S. Pat. Nos. 3,457,359 to Soucy, 3,520,110 to Knauer, and 4,215,384 to Elson. In situations where the fuel supply nozzle is not electrically conductive, such as an insulated or polymeric nozzle, it is known to provide an electrically conductive connection between the fuel inlet port and the fuel source through such device as shown in U.S. Pat. No. 4,005,339 to Plantard.
As with grounding systems for tank trucks or other fuel transport vehicles, these grounding systems for aircraft rely upon an operator to properly complete the grounding connections to the aircraft prior to initiating fueling. This step may be forgotten or disregarded when the operator is poorly supervised, distracted, or rushed. Moreover, connecting the grounding wires of the fuel supply hose in close proximity to the open fuel inlet of the reservoir prior to connecting the aircraft grounding wire may still produce a electrostatic discharge that will ignite fuel or vapors remaining within the reservoir. Because of these risks associated with fueling operations, current Federal Aviation Administration guidelines require fueling to be performed by fueling stations whose operators have met minimum training or certification standards, and for which some on-site supervisory authority is therefore usually present. However, these training standards and the presence of on-site supervisory authority do not ensure compliance with proper grounding protocols by fueling station operators, and those guidelines stand as an impediment to designing more effective self-service fueling systems.
It is similarly known in military and commercial aviation maintenance applications to use a triangular bonding configuration whereby both the aircraft and equipment are grounded to a common ground and to one another. However, as with conventional manual grounding systems, the triangular bonding method relies on the operator to ensure that proper grounding contacts are attained. It is further known to incorporate time delays on fuel dispensing and pumping systems that utilize leak detection equipment, in order to maintain the solenoid valves in a closed position for a predetermined time until the leak detection equipment initially stabilizes and the pump reaches operating pressure. The time delay systems do not respond to a specified condition such as proper grounding of the aircraft or vehicle, but rather automatically engage the fuel dispensing system after a preset time has elapsed.
U.S. Pat. No. 2,767,659 to Greenblatt discloses a device for automatically grounding tank trucks using electrically conductive spring members that contact the undercarriage of a tank truck when it is properly positioned for fuel transfer and dissipate accumulated electrostatic charge. The device includes a contact assembly that can be used to disable the fuel delivery system until the spring members are depressed by contact with the vehicle.
However, systems such as the Greenblatt '659 device require significant uniformity in the dimensions of the tank trucks and the structure of their conductive undercarriages, and are therefore unsuitable for non-commercial vehicles, and other applications such as aircraft or marine vessels. Moreover, systems such as Greenblatt '659 respond to physical contact, and while the spring members and associated intrinsic linkages may be properly grounded, there is no certainty that the surface of the undercarriage will form an electrically conductive contact with the spring members.
Various types of electronic circuits (such as ground fault indicators and the like) can be utilized to indicate the electrically grounded condition of the circuit itself or an external object. U.S. Pat. No. 2,660,717 to Hood discloses a common-point rectifier circuit used to sense a live grounded connection in a conductive device used in a surgical operating room or industrial plant, and alternately to drain accumulated static charge away from a patient or workpiece. U.S. Pat. No. 2,611,862 to Riddle discloses a passive bridge rectifier circuit having tuned and balanced impedances on opposing sides of the bridge, with the bridge becoming unbalanced and activating a relay or signal lamp when the grounding cable connected to one side of the bridge is attached to an external object having an impedance. Riddle '862 further suggests using the circuit to prevent loading of a tank if the grounding cable is not connected. U.S. Pat. No. Re. 25,957 to Caldwell discloses a simple bridge rectifier circuit and signal lamp connected to a grounding cable which is used to ground a tank truck or similar object.
While suitable as status indicators in some applications, these circuits present several drawbacks for use in grounding systems associated with fueling operations, and particularly with dual grounding systems such as for aircraft. Hood '717 is designed to detect and identify a grounded current sufficient to cause muscular contraction or reflex if that current is transmitted from the device through a body, rather than passively indicating the grounded condition of the device. A circuit such as Riddle '862 can identify whether the grounding cable has been attached to some object using a much lower RF signal, but since the bridge will be unbalanced by attaching the cable to grounded or ungrounded objects of almost any impedance, the circuit will not identify whether the grounding cable has been attached to the particular object desired to be grounded. Caldwell '957 requires a two-conductor grounding cable, with the opposing jaws of the clamp acting as the poles of a simple switch to simultaneously complete the grounding connection and unbalance the bridge circuit when the clamp is connected to any electrically conductive object.
These types of ground indicating circuits are relatively complex and expensive to assemble, and require significant tuning and adjustment to operate properly. The circuits can be sensitive to adverse environmental conditions and damaged by overloads. Moreover, such circuits may be easily circumvented if an operator wishes to intentionally disregard grounding an aircraft in compliance with an established fueling protocol, or as a result of other equipment tampering. This can be particularly hazardous in designing self-service fueling systems, since once circumvented the circuit will remain inactive as a ground status indicator or protocol reminder for subsequent operators, and it is anticipated that many operators will use the self-service fueling system between inspections by a supervisory authority.